Waveguide impedance matching transitions while maintaining effective cross-section unchanged



March 1965 F. SCHELISCH ETAL 3,176,249

WAVEGUIDE EDANCE MATCHING TRANSISTIONS WHILE MAINTAINING EFFECTIVE CROSS-SECTION UNCHANGED Filed Oct. 7. 1960 A'rvoa'vem United States Patent 3,176,249 WAVEGUIDE IMPEDANCE MATCHING TRAN- SITIONS WHILE MAINTAINING EFFECTIVE CROSS-SECTION UNCHANGED Ernst Friedrich Schelisch, Great Baddow, and John Alfred Berry, Chelmsford, England, assignors to The Marconi Company Limited, a British company Filed Oct. 7, 1960, Ser. No. 61,269 Claims priority, application Great Britain, Nov. 30, 1959, 40,577 59 8 Claims. (Cl. 333-34) This invention relates to Waveguides and more specifically to circular sectioned waveguides, for the transmission of TE waves (where n is any whole number, usually unity).

- It is well known to make a circular waveguide in the form of a helix of metal wire or strip within a sheath or in the form of a series of spaced annular rings within a sheath or of corrugated wall construction. Constructions of this type are periodic or near-periodic structures with rotary symmetry about the axis and are commonly adopted inorder to provide a. predetermined surface impedance for the H wave to be propagated, vso as to secure suppression or part suppression of unwanted modes, or to enable the wave to be guided round a bend without serious losses or to provide a flexible guide. In sheathed structures of this nature the sheath is usually of synthetic material. However, difficulties arise when a guide of this nature-hereinafter termed, for brevity, a flexible guide, though it will be understood that the degree of flexibility is not necessarily high-is required to be joined end to end to a more ordinary, solid wall type of circular guide. These difficulties arise because the surface impedance of a flexible guide is substantially different from that of a solid wall guide and accordingly reflections and/or mode conversions are apt to occur where the two guides adjoin one another.

According to this invention at least one of two differing circular sectioned waveguides having different surface impedances over the major parts of their lengths and which are to adjoin end to end is varied in internal dimensions over a portion of its length terminating in one end in such manner as to provide a gradual transition of surface impedance from the value over the major portion of the length of said guide to another, different predetermined value of surface impedance at said end. The term surface impedance, which appears throughout the following description and in the appended claims, is to be understood to mean the impedance presented to the TE mode as a result of the configuration of the internal wall surface of the waveguide under consideration.

According to a feature of this invention a solid walled circular sectioned waveguide is formed near one end with internal grooving of dimensions which increase towards said end so as to present a gradual variation of surface impedance from the value presented by the ungrooved portion of said guide to a predetermined value at said end. In the simplest and preferred case the internal grooving is in the form of internal ring grooves which are side by side and of a depth and/or width which increases, either from groove to groove, or from group of grooves to group of grooves, towards said end of the guide. It is also possible to make the grooving in the form of a screw thread of increasing depth and/ or width towards said end of the guide. By choosing the predetermined value of surface impedance at the end of the guide to match the surface impedance presented by an adjoining flexible guide, a run of waveguide consisting of the combination of a length of solid walled guide with internal grooving as above set forth, and a length of flexible guide mounted end to end, may be constructed and will be substantially free from reflection or mode conversion at the juncture of the two guides.

It is possible, though not preferred, tovary the surface impedance of a flexible guide near one end thereof so that it can be satisfactorily joined end to end to an ordinary solid walled circular guide without causing appreciable reflection or mode conversion. This method of carrying out the invention is not preferred because it is more expensive than the method in which variation of surface impedance of the solid walled guide is effected. However, the method is practicable and according to another feature of the invention the helix of a helical guide consisting of a metal helix in a sheath is formed of metal wire or strip, the cross-sectional dimensions of which are reduced, over a length near one end of the guide, from the cross-sectional dimensions employed over the major part of the length of said guide to predetermined smaller cross-sectional dimensions chosen to provide a predetermined surface impedance at said end of the guide. In the simplest and usual case in which the helix is of round Wire, the diameter of the wire is reduced, either by tapering or in steps, towards said end of the guide.

The invention is illustrated in the accompanying drawings in which FIG. 1 is a half sectioned simplified representation of a preferred embodiment, and FIGS. 2 and 3 are similar representations of the modifications.

Referring to FIG. 1, two lengths 1 and 2 of waveguide adjoin end to end at 3 to form a continuous run. The adjacent ends of the guides may be held together in any convenient way, and, for simplicity in drawing, a sleeve 4 extending over both guide ends is indicated as provided for this purpose. The guide 2 is a helical guide of known construction and consists of round metal wire 2a wound into a helix and externally sheathed by a covering 217 of synthetic material. The other guide 1 is a solid walled metal tube. In accordance with this invention this tube is internally grooved as indicated at In by grooves which are shown as of triangular section and which are of progressively increasing depth and Width towards the junction 3. This grooving of progressively varied dimensions is designed in accordance with known principles to produce a smooth transition in surface impedance from the value presented by the guide 1 over the ungrooved portion of its length to a value which substantially matches the surface impedance of the guide 2 at the junction plane 3.

FIGURE 2 shows a modification which is not preferred because it is more expensive than the embodiment of FIGURE 1. It is, however, practical. FIGURE 2 needs little description because like references are used for like parts in both figures. As will be apparent from FIGURE 2, the guide 1 is not internally grooved but is left as a simple solid walled tubular guide of ordinary construction. Instead of grooving the guide 1, the wire helix of the guide 2 is modified towards the junction plane 3 by being made of wire 20 which is reduced in diameter either in tapered fashion or in steps towards the said plane 3 so that the surface impedances of the two guides where they adjoin are as nearly as possible the same.

The invention may be usefully employed in all cases where two guides of different constructions and different surface impedances over their general lengths are required to be joined end to end, for example where a solid walled tubular guide is required to be joined to a helical guide as illustrated or to a flexible guide of the kind made from spaced annular rings or of the so-called corrugated wall construction.

FIGURE 3 shows a modification wherein a solid walled tubular guide 1 is joined end to end to a flexible guide of the kind made from spaced annular rings. As shown the guide 2 is built up of metal rings 2d spaced by rings 2e However, the grooving may also be the same as shown' in FIGURE 1 if desired. Alternatively gradual varlation,

of surface impedance may be veffectedin the guide 2 by gradually changing the spacings between the metal rings 2d (i.e., ,by progressively varying the thickness of the rings 2e) or both these expedients may be adopted.

The required gradually variation of: surface impedance, whether effected in the solid walled guide or in the flexible guide, or in both, may be made, by suitable design in the gradual variations of the form of groups which areside by side and of a depth which increases from each group of grooves to the next group of grooves towardsaid one'end of the first waveguide I a 5.. An arrangement as claimed in claim'2 Whereinsaid grooves are internalring grooves disposed side by side,

said grooves 'having' a width which progressivelyincreases from groove to groove toward said one end of the first Waveguide.

6. An arrangement as claimed in claim 2 wherein said grooves are internal ring grooves, said grooves being in the form of groups which are side by side and of a .Width which increases from each group of grooves to the next 7 group of grooves toward said one end of the first waveaccordance withknown principles, to hem accordance with any desired law of variation, for example, a linear 1 law of variation 'or a binomial variation. We claim:

or an exponential law of 1. A waveguide coupling arrangement comprising, 'in

'combinaton: a first waveguide of circular, cross-section having a predetermined value of surface impedance over the major portion of its length, the inner surface of said.

' reduced cross-sectional dimensions over the remaining first waveguide having plural circumferential grooves therein over the remaining portion of its length terminate ing in one end to provide a gradual transition of surface impedance for matching said predetermined value to another, different value of surface impedance at said end, said grooves having dimensions which'vary progressively toward said end, and both said major portion and the" remaining portion having the same constant minimum internal diameter; a second Waveguide of'circular cross;

section having a surface impedance substantially equal to saiddifierent value; and coupling means for joining said first waveguide and said second waveguide end to end for propagation of energy in the TEb mode between the two'waveguides without any reflection or-mode conversion.

2; An arrangement as claimed in claim 1 wherein said first waveguide has a solid wall, and whereinthe dimensions of said grooves continuously increase toward said one end. a a V 3. An arrangement as claimed inclaim 2 wherein said grooves are internal ring grooves disposed side by side, said grooves having a depth which progressively increases from groove to groove toward said one end of the first waveguide. i 4. An arrangement as claimed in claim 2 wherein said guide. w I I 7. A helical Waveguide: for propagating'energy in the TE rmode comprising, in combination: a metal helix having predetermined cross-sectional dimensions over the major portion of the length of said waveguide, said major portion having a predetermined value" of surface impedance, saidhelix having an end section of progressively portion of the lengthof said'waveguide terminating in one end to provide a gradual transition of surface im- "pedance for matching said predeterminedvalue'to another, difierent valueofsurface impedance at said end,

both said" major portion and-said remaining portion having the. same constant minimum internal diameter; and a sheath of synthetie material surroundingsaid metal helix. a I

; 8. A helical waveguide 'as claimed in claim 7 wherein the metal helix-is of round wire and the diameter of said wire is progressively reduced toward said one end of the waveguide. 5

l References Cited by the Examiner UNITED STATES PATENTS 2,600,169 7, 6/52; Lamb, 333 9s 2,659,817 r 11/53 Cutler, '333 95 2,751,561 6/56 King 333-98 2,848,695 .8/58 Pierce 333 95 I 2,865,008. 12/58 Kock 333-95 2,891,190 6/59 COhil 333 -95 ,966,643 12/60- Kohma'n 333-95 I p FOREIGN PATENTS 1 812,032 4/59 Great Britain. "823,565 11/59 Great Britain. 883,439 11/6 1 Great Britain.

HERMAN KARL ,SAALBACH, Primary Examiner.

grooves are internal. ring grooves, said grooves being in ELI'J. SAX, Eraminer. 

1. A WAGEGUIDE COUPLING ARRANGEMENT COMPRISING, IN COMBINATION: A FIRST WAVEGUIDE OF CIRCULAR CROSS-SECTION HAVING A PREDETERMINED VALUE OF SURFACE IMPEDANCE OVER THE MAJOR PORTION OF ITS LENGTH, THE INNER SURFACE OF SAID FIRST WAVEGUIDE HAVING PLURAL CIRCUMFERENTIAL GROOVES THEREIN OVER THE REMAINING PORTION OF ITS LENGTH TERMINATING IN ONE END TO PROVIDE A GRADUAL TRANSITION OF SURFACE IMPEDANCE FOR MATCHING SAID PREDETERMINED VALUE TO ANOTHER, DIFFERENT VALUE OF SURFACE IMPEDANCE AT SAID END, SAID GROOVE HAVING DIMENSIONS WHICH VARY PROGRESSIVELY TOWARD SAID END, AND BOTH SAID MAJOR PORTION AND THE REMAINING PORTION HAVING A THE SAME CONSTANT MINIMUM INTERNAL DIAMETER; A SEOND WAVEGUIDE OF CIRCULAR CROSSSECTION HAVING A SURFACE IMPEDANCE SUBSTANTIALLY EQUAL TO SAID DIFFERENT VALUE; AND COUPLING MEANS FOR JOINING SAID FIRST WAVEGUIDE AND SAID SECOND WAVEGUIDE END TO END FOR PROPAGATION OF ENERGY IN THE TE01 MODE BETWEEN THE TWO WAVEGUIDES WITHOUT ANY REFLECTION OR MODE CONVERSION. 